In the diagram below, DE is parallel to AB in right triangle ABC. DE bisects AC and CB. If AD = 3, and CB = 5, what is the length of DE?
А
D

The length DE is the hypotenuse of triangle CDE
The length DE is [tex]\frac{\sqrt{61}}2[/tex]
The given parameters are:
[tex]AD =3[/tex]
[tex]CB = 5[/tex]
Given that line DE bisects AC and CB, it means that:
[tex]CD = AD = 3[/tex]
[tex]CE = EB = 2.5[/tex] i.e. CB/2
So, we have:
[tex]DE =\sqrt{AD^2 + CE^2[/tex] --- Pythagoras theorem
Substitute known values
[tex]DE =\sqrt{3^2 + 2.5^2[/tex]
[tex]DE =\sqrt{15.25[/tex]
Express as fraction
[tex]DE =\sqrt{\frac{61}{4}[/tex]
Simplify
[tex]DE =\frac{\sqrt{61}}2[/tex]
Hence, the length DE is [tex]\frac{\sqrt{61}}2[/tex]
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